The genomes of the cereal powdery mildew Fungi, Blumeria graminis

Abstract

Blumeria graminis causes powdery mildew disease on several important small grain cereals and several grasses. These ascomycete fungi, like all members of the Erysiphales (Leotiomycetes), are obligate biotrophs. They have a distinct life-cycle characterized by a mainly epiphytic growth habit on the epidermis of leaves and stems. The fungi penetrate the epidermal cells only to form haustoria surrounded by a host membrane. The Blumerias occur as distinct formae speciales that only infect one host species. The polycyclic asexual phase is most commonly observed; here the airborne conidia, which form the eponymous powder, are responsible for the dispersal and infection. The rarer sexual cycle usually occurs once a year, at the end of the host growing season. The size of the Blumeria genomes, over 120 Mb, is much larger than those of related ascomycetes. This is due to an extraordinary expansion of the retrotransposons evident as repetitive DNA. At the same time, the protein coding gene complement is reduced to about 6500: gene families are smaller, there are fewer paralogs; some metabolic pathways are missing altogether, notably those responsible for nitrate and sulfate metabolism. In the face of this large scale reduction in gene number, we observe a massive expansion of genes encoding small secreted proteins that are candidate effectors of pathogenicity. Understanding the role played by these candidate effectors, which constitute over 8 % of the total genes, is a challenge for research in the coming years.